As the most popular one of fiber-reinforced composite materials, fiber-glass-reinforced resin, which is fiber glass impregnated with resin, has been known. In general, fiber-glass-reinforced resin is nontransparent. Methods for obtaining transparent fiber-glass-reinforced resin, in which the refractive index of fiber glass and that of the resin matrix are matched, have been disclosed in patent documents 1 and 2.
Incidentally, transparent flexible substrates used for implementation of LED or organic electronics devices are required to have properties such as a weak tendency for thermal expansion as well as a high strength, a high elasticity, and a light weight. However, fiber-glass-reinforced resin substrates can have a weak tendency for thermal expansion and a high strength but cannot have a light weight. Also, in the ordinary way of fiber glass reinforcement, the fiber diameter is on the order of microns, and thus resultant substrates can be transparent only at a specific atmospheric temperature and for a specific wavelength range, and transparency is insufficient in practical settings. Furthermore, changes in atmospheric temperature may affect flatness and surface smoothness.
Patent document 3 mentioned below describes a flexible fiber-reinforced composite substrate material that is excellently transparent regardless of temperature, range of wavelength in the visible range, or the refractive index of the resin material used in combination therewith, is excellent in terms of surface smoothness, has a weak tendency for thermal expansion as well as a high strength and a light weight. This fiber-reinforced composite material contains fiber having an average fiber diameter in the range of 4 to 200 nm and a matrix material, and the transmittance for rays of light having a wavelength in the range of 400 to 700 nm calculated for a thickness of 50 μm is equal to or higher than 60%.
For improved hygroscopicity of this fiber-reinforced composite material, hydroxy groups of cellulose fiber, a constituent of the fiber-reinforced composite material, are chemically modified; the resultant fiber-reinforced composite material is described in PATENT DOCUMENT 4.
In patent documents 3 and 4, cellulose fiber produced by bacteria (hereinafter, referred to as “bacteria cellulose”) or cellulose fiber obtained by unbraiding pulp, cotton, or some other similar material into microfibrils is processed into a sheet, and then the sheet is impregnated with a matrix material.
Also, patent documents 5 and 6 have proposed ultrafine fiber obtained by suspending cellulose fiber or some other kind of naturally occurring fiber and then processing the suspension between two rotating discs for unbraiding. In these literatures, patent documents 5 and 6, fiber is fragmented by mechanical unbraiding cycles repeated 10 to 20 times.
To obtain a highly transparent fiber-reinforced composite material by impregnation of a fine-fiber sheet with a matrix material, such as those described in patent documents 3 to 6, it is required that the fiber constituting the sheet be fragmented into sufficiently small pieces (nanofiber). And, to obtain a fiber-reinforced composite material with a high modulus of elasticity and a low coefficient of linear thermal expansion, it is required that cellulose crystals constituting the fiber cannot be broken by unbraiding and keep its high degree of crystallinity even after unbraiding.
To this end, in patent document 6, the precursor of nanofiber is conditioned before unbraiding to contain water at a predefined content ratio so that it can be prevented from drying; as a result, a nanofiber sheet is obtained with sufficiently small fiber pieces.
Patent document 1: Japanese Unexamined Patent Application Publication No. 9-207234
Patent document 2: Japanese Unexamined Patent Application Publication No. 7-156279
Patent document 3: Japanese Unexamined Patent Application Publication No. 2005-60680
Patent document 4: Japanese Unexamined Patent Application Publication No. 2007-51266
Patent document 5: Japanese Unexamined Patent Application Publication No. 2003-155349
Patent document 6: Japanese Unexamined Patent Application Publication No. 2008-24788